Mirror actuator in BGE
You can scale the object with negative value along a single axis to get a mirror.

To do so, play an action with scale keyframes:


frame 1: scale = (1,1,1)
frame 2: scale = (-1,1,1) [depends on your mirror axis]


Hint: As this is no turning, you still will see the front side when flipping. This means you can keep Backface Culling enabled.